A number of analytical instruments, such as spectrophotometers, are currently available to measure the reflectance of various materials, usually on a wavelength-by-wavelength basis. In principle, reflectance of a material can vary from 0% (black) to 100% (white). However, real materials never really reach these ideal values, and the reflectance range of actual materials typically varies from a low of 1–2% to a high of approximately 99%.
In order to accurately measure absolute reflectance of a test material, the analytical instrument has to be properly calibrated. Although it is possible to use a highly reflective (>90% reflective) standard and a highly absorptive (>90% absorptive) standard, which are readily available, it is not the best way to perform those calculations because the calculated reflectance is subject to errors caused by possible non-linearity of response of the instrument. It is preferable, therefore, to bracket, as closely as possible, the reflectance of the unknown material with calibration standards of known reflectance. The possible non-linearity will have a smaller range in which to act and, therefore, the results can be more accurate. This can be achieved by using standards having reflectance between the ends of the reflectance spectrum, commonly known as “gray” standards.
In order to produce different “gray” standards, it has been suggested to combine materials having various degrees of reflectance, the amount of each dependent on the desired reflectance of the final standard. There are various “gray” standards having a wide range of reflectance available in the marketplace.
However, one problem with these standards is that the materials used to prepare them are not sufficiently homogeneous. For example, some new instrumentation uses imaging microscopes to measure the reflectance spectra of hundreds or thousands of microscopically-sized spots on a sample and thereby determines what has come to be called a “hyperspectral image”. Therefore, the use of non-uniform material has resulted in a situation where a nominally “gray” standard is actually resolved into regions with varied reflectance, none of which can be calibrated for their absolute reflectance.
In addition, the presence of unusually light or unusually dark regions in a standard can potentially affect the readings produced by instruments (including older spectrometers) that observe a large area of a sample when high-precision measurements are attempted, due to the light or dark spots moving into or out of the field of view of the instrument.
Another need for a uniformly “gray” standard arises even when absolute reflectance values are not needed. For example, an extremely uniform “gray” standard is essential to characterizing the relative response of different detectors in a multi-detector array.
What is desired, therefore, is a calibration standard formed from “gray” material that is much more homogeneous through the visible and near-infrared regions of the spectrum than the materials currently used. What is also desired is the method for calibrating optical instruments using these standards.